1. Field of the Invention
This invention relates, in general, to voltage generators and more particularly to an integrated circuit bandgap voltage reference circuit that provides a reference voltage (V.sub.BB) and current source voltage (V.sub.CS).
2. Background Art
Many types of bandgap voltage reference circuits are known in the art. Typically, an output voltage having substantially zero temperature coefficient is produced by summing two voltages having opposite temperature coefficients, i.e., one positive temperature coefficient and one negative temperature coefficient.
In general, the positive temperature coefficient is produced by using two transistors operated at different current densities as is well understood. By connecting a resistor in series with the emitter of the transistor that is operated at a smaller current density and then coupling the base of this transistor and the other end of the resistor across the base and emitter of the transistor operated at the higher current density produces a delta V.sub.BE voltage across the resistor that has a positive temperature coefficient. This positive temperature coefficient voltage is combined in series with the V.sub.BE of a third transistor which has a negative temperature coefficient in a manner to produce a composite voltage having a very low or zero temperature coefficient. These prior art voltage reference circuits are generally referred to as bandgap voltage references because the composite voltage is nearly equal to the bandgap voltage of silicon semiconductor material, i.e., approximately 1.2 volts. Typically, the two transistors of the bandgap cell are NPN devices with the first transistor having an emitter area that is ratioed with respect to the emitter area of the second transistor whereby the difference in the current density is established by maintaining the collector currents of the two transistors equal.
One particular previously known circuit as described in U.S. Pat. No. 3,617,859, which is described further in the Detailed Description of the Invention, comprises a Widlar circuit having two NPN transistors scaled as described above and having a positive temperature coefficient. These two NPN transistors drive the base of a third NPN transistor having a negative temperature coefficient. A PNP transistor shunts, and therefore controls, current through the third NPN transistor to compensate for variations in the voltage supply V.sub.EE. However, use of a PNP transistor thusly presents difficulties as follows. There are three types of PNP devices that are generally available in high speed ECL bipolar technologies. These are the vertical/substrate PNP device, which utilizes no N+ buried layer as part of the base region, a similar PNP device which does utilize the N+ buried layer as part of the base region, and finally a lateral PNP device. Traditionally the first type of PNP device has been used effectively as the shunt transistor because of its high beta characteristics. However, the vertical dimensions of advanced processes (i.e., epitaxial thickness) have been reduced over time due to speed and packing density requirements. This tends to cause this type of PNP device to have a very high beta, which in turn, results in a susceptibility to low V.sub.CEO (collector-emitter voltage with the base open) breakdown and collector-emitter punch through shorting. This is due to variable epitaxial thickness and variable auto-doping during epitaxial growth. Hence, this type of vertical/substrate PNP device has become unreliable in advanced high speed processes. By adding an N+ buried layer to this type of device, the beta is reduced to a typical value less than one. Thus, the second type of PNP device is unacceptable as a shunt device. The lateral PNP device arranged in a donut shaped layout configuration can produce a beta in the five to twenty range. However, this beta is highly variable over process due to poor epitaxial doping control and poor base width control. Also, these devices have poor current density capability (i.e., low current beta rolloff) due to the low base doping. Therefore, they need to be made very large to satisfy the current carrying requirements of a shunt device and require a large area on the chip.
Thus, practical low voltage bandgap reference circuits cannot be manufactured utilizing present day high speed, low voltage semiconductor processes because of the poor quality of the PNP transistors.
Hence, a need exists for a low voltage reference circuit for providing a bandgap reference voltage having excellent temperature performance, power supply rejection and load regulation that does not require a PNP transistor for controlling the current within the NPN transistor having a negative temperature coefficient.